How to align duplication correct way
With the curve selected, under the Curve tab of the properties area, activate Follow.



Go into Edit Mode and change the curve's Tilt so that the curve's arrows are perpendicular to the shoe geometry. With a control point selected use Curve > Control Points > Tilt or Ctrl+T to do that. I suggest enabling Proportional Editing to achieve a smooth transition. (The tilt can also be changed after the diamonds are aligned, which probably will make errors more obvious.)



Change the diamonds' Rotation so that the diamonds are aligned with the surface.
XYZ angles of 0°, -15° and -90° seem to work fine.



To make sure that the curve is running at the surface of the shoe, add a Shrinkwrap Modifier with the shoe as target, mode being Nearest Surface Point and enable Apply on Spline.



You might have to move some of the curve's control points outwards so that the shrinkwrap modifier doesn't snap the diamonds to the inside of the shoe.



Final blend file: